Silicon carbide densified by self-sintering, also called pressureless sintering, is very hard, has good corrosion and abrasion resistance and high thermal conductivity. Self-sintered silicon carbide is used for sliding face applications, such as pump seals and bearings, in environments where such seals and bearings are exposed to acidic, caustic, corrosive or abrasive substances. However, self-sintered silicon carbide is not self-lubricating. Thus, if self-sintered silicon carbide is used in a seal or bearing face which runs against a face of another hard, non-self-lubricating material, such as self-sintered or reaction-bonded silicon carbide, the seal or bearing faces must be exposed to a lubricating fluid or used in fluid applications, such as in liquid pumps. The fluid provides a film between the sliding faces which lubricates the surfaces, reduces friction and prevents failure.
Self-sintered silicon carbide, when run against other hard, non-self-lubricating materials, is vulnerable to catastrophic failure when exposed to rapid temperature changes or if an insufficient amount of lubricating fluid is present, such as under upset conditions in pump applications when a pump accidently runs dry or if the pump is energized prior to connection of the fluid stream. Insufficient lubrication can cause the self-sintered silicon carbide to explode in the most severe conditions. Further, self-sintered silicon carbide used in sliding face applications against other non-self-lubricating materials exhibits high wear rates when under demanding conditions such as high speed and contact pressure.
Porous self-sintered silicon carbide also is not self lubricating, and has the disadvantages of self-sintered silicon carbide discussed above. Although, the surface pores of porous self-sintered silicon carbide can help the material to retain some lubricating fluid during use, the materials cannot be run dry. Thus, like self-sintered silicon carbide, porous self-sintered silicon carbide must be used in fluid applications where a lubricating fluid is provided.
Further, because the pores of porous self-sintered silicon carbide are not interconnected, porous self-sintered silicon carbide cannot be impregnated with resin, carbon, TEFLON, metals or other compounds or materials.
Siliconized graphite, i.e., graphite siliconized by chemical vapor reaction (CVR), has some self-lubricating properties and has good wear characteristics compared to self-sintered silicon carbide and other silicon carbide composites. Siliconized graphite is produced from specially formulated graphite which is reacted with silicon monoxide (SiO) vapor. The chemical vapor reaction produces a silicon carbide layer (typically 40 mm thick) on an underlying graphite substrate. The silicon carbide surface layer typically has a microstructure of graphite inclusions and interconnected pores throughout the surface layer. A lubricating substance can be impregnated in such pores to produce a self-lubricating material suitable for bearings and seals having a degree of survivability in dry-running conditions.
Siliconized graphite, however, has significant drawbacks. Because the self-lubricating silicon carbide structure is a thin surface layer, seals, bearings and other sliding face products made from siliconized graphite cannot be lapped, polished or repaired by a customer without risking penetration of the silicon carbide layer into the relatively soft graphite substrate. Also, the silicon carbide layer is prone to cracking, delamination and other defects. Further, siliconized graphite is very difficult to impregnate with carbon. Because the silicon carbide surface layer is thin, very porous and bonded to a relatively soft graphite substrate, the silicon carbide layer is weak. When the silicon carbide layer is impregnated with a carbon precursor, such as resin, and the precursor is carbonized, volatilization of the precursor usually causes the silicon carbide layer to crack, delaminate, break apart, or in some cases explode.